Load cell with strain gauges having low temperature dependent coefficient of resistance

ABSTRACT

A load cell utilizable in a weighing apparatus. The load cell is fabricated by forming a strain gauge pattern (4) of a copper-nickel alloy on a surface of a strain inducing element (1) by the use of a sputtering technique and subsequently heat-treating the strain gauge pattern under an oxygen-free atmosphere, wherein the weight ratio of copper and nickel is chosen to be of a value effective to permit the temperature dependent coefficient of resistance of the strain gauge pattern (4), which undergoes shrinkage and expansion together with the strain inducing element (1), to be substantially zero. In this way, the load cell can be obtained which is not affected by a change in temperature and which gives a high measurement precision.

FIELD OF THE INVENTION

The present invention relates to a load cell of a type used in aweighing apparatus such as, for example, an electronic scale, comprisinga strain gauge formed by the use of a sputtering technique on a surfaceof a strain inducing element.

BACKGROUND ART

It is well known that a load cell comprising a strain gauge formed bythe use of a vapor deposition technique or a sputtering technique ismanufactured by applying a heat-resistant high molecular material suchas, for example, polyimide, on a surface of a strain inducing element toform an electrically insulating film, subsequently forming a thin filmof nickel-chromium alloy or tantalum nitrate on a surface of theelectrically insulating film by the use of a vapor deposition techniqueor a sputtering technique and finally photo-etching the thin film toform a resistance pattern.

The load cell particularly used in a weighing apparatus such as a scalecomprises four strain gauges which are electrically connected to form abridge circuit. Therefore, where all of the strain inducing elementsvary uniformly with a change in temperature, temperature dependentchanges in outputs from the respective strain gauges can becounter-balanced with each other. However, where an article of atemperature considerably different from a room temperature such as, forexample, a frozen food material, is placed on a load supporting plateconnected with one end of the load cell, a temperature gradient occursin strain inducing elements to such an extent as to result in varyingresistances of the respective strain gauges according to a temperaturedependent coefficient of resistance. Once this occurs, the variations inresistance among those strain gauges can no longer be counterbalancedwith each other by the bridge circuit.

By way of example, in the case of the load cell comprising strain gaugesprepared by forming an electrically insulating film of polyimide on asurface of a strain inducing element and then sputtering a thin film oftantalum nitrate on a surface of the electrically insulating film, oneof the resultant strain gauges may have a temperature dependentcoefficient of resistance which is about -40 PPM/deg(°C.) and,consequently, when this load cell is applied in a weighing apparatus, aproblem would occur in that an measurement error in the order of about200% per degree at maximum tends to occur.

The present invention has been devised with the foregoing problem takeninto consideration and has for its object to provide a load cell whereinstrain gauges each having an extremely small temperature dependentcoefficient of resistance are integrally formed.

DISCLOSURE OF THE INVENTION

In order to solve the foregoing problem, in the practice of the presentinvention, in a load cell fabricated by forming a strain gauge patternfrom a film of a copper-nickel alloy formed on a metallic straininducing element by the use of a sputtering technique and subsequentlyheat-treating the strain gauge pattern under an oxygen-free atmosphere,the weight ratio of copper and nickel is chosen to be of a valueeffective to permit the temperature dependent coefficient of resistanceof the strain gauge pattern, which undergoes shrinkage and expansiontogether with the strain inducing element, to be substantially zero,that is, a value effective to counterbalance a change in resistanceresulting from the shrinkage and expansion of the strain gauge due tothe linear expansion of the strain inducing element and the temperaturedependent coefficient of the copper-nickel alloy used to form the straingauges with each other.

The strain inducing element referred to above may be made of aluminumalloy, stainless steel, brass, phosphoric bronze, low alloyed steel orcarbon steel.

By way of example, where the strain inducing element referred to aboveis made of aluminum alloy, stainless steel or brass, and if thecoefficient of linear expansion of this strain inducing element isexpressed by β, the content (wt %) of nickel in the copper-nickel alloyused as material for the strain gauge pattern is preferred to be:##EQU1##

According to the present invention, the change in resistance resultingfrom the shrinkage and expansion of the strain gauge due to the linearexpansion of the strain inducing element and the temperature dependentcoefficient of the copper-nickel alloy used to form the strain gaugesare counterbalanced with each other and the change in resistanceresulting from a change in temperature of any one of the strain inducingelement and the strain gauges can be minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(I) to FIG. 1(III) are explanatory diagrams showing a method ofmanufacturing a load cell according to the present invention;

FIG. 2 is a longitudinal sectional view showing one example of a highfrequency magnetron sputtering apparatus utilizable in the formation ofa resistance film;

FIG. 3 is a perspective view showing a load cell according to oneembodiment of the present invention;

FIG. 4 is a graph showing a relationship between a composition of acopper-nickel alloy, which is employed when an aluminum alloy is usedfor a strain inducing element, and a temperature dependent coefficientof resistance of a strain gauge formed on the strain inducing element;

FIG. 5 is a graph showing the rates of change in resistance andtemperature exhibited by the load cell of the present invention and aprior art load cell;

FIG. 6(I) to FIG. 6(III) are perspective views of respective targetsused to vary composition ratios during the formation of a resistancefilm; and

FIG. 7 is a perspective view showing a different example of straininducing element.

BEST MODE FOR CARRYING OUT THE INVENTION

The details of the present invention will now be described withreference to the accompanying drawings showing one embodiment thereof.

FIG. 1 is a diagram showing the sequential steps of manufacture of aload cell according to the present invention. In this figure, referencenumeral 1 represents a strain inducing element made of an aluminum alloyhaving an excellent spring characteristic, said strain inducing elementhaving a surface on which strain gauges are to be formed. The surface ofthe strain inducing element 1 is coated by the use of, for example, aspinner with a heat-resistant high molecular material such as, forexample, polyimide to a thickness of 2 μm thereby to form anelectrically insulating film 2((I) in FIG. 1).

Then, a thin film 3 of a copper-nickel alloy having a thickness of about0.1 to 1 μm is formed on a surface of the electrically insulating film 2((II in FIG. 1).

FIG. 2 illustrates one example of a high frequency magnetron sputteringapparatus which is used for the formation of the thin film, whereinreference numeral 10 represents a vacuum vessel having a gas inflow port11, communicated with a source of supply of a working gas, and a gasoutflow port 12 communicated with an exhaust means. The vacuum vessel 10includes a substrate holder 13 positioned in an upper region of theinterior thereof and into which a cooling water is supplied, and ashield plate 14 positioned in a lower region of the interior thereof.Positioned atop the shield plate 14 is an electrode 16 whichconcurrently serves as a target holder plate and into which a coolingwater is supplied, said electrode 16 having plasma catching magnets 15positioned beneath the undersurface thereof. This electrode 16 iscoupled with a high frequency electric power source 18 through amatching box 17.

Using the above described apparatus, the strain inducing element 1referred to hereinbefore ((I) in FIG. (1)) is fixed on the substrateholder 13 with the electrically insulating film 2 oriented towards theelectrode 16, and a target material S made of a copper-nickel alloy isfixed on the electrode 16. Thereafter, the vacuum vessel 10 is evacuatedwith air inside the vacuum vessel 10 exhausted to the outside and, then,an argon gas is introduced into the vacuum vessel 10 to 3 to 30×10⁻³Torr.

When the above described preparation has been completed, while thetemperature of the strain inducing element 1 is kept at 50° to 180° C.,a high frequency electric power of 13 MHz is supplied at 100 to 1,000 Wto form a resistance thin film 3 of copper-nickel alloy having a filmthickness within the range of 0.1 to 1 μm ((II) in FIG. 1).

The strain inducing element having the thin film of copper-nickel alloyso formed thereon is removed out of the vacuum vessel 10, followed by anapplication of a photo-resist on a surface of the resistance thin film3.

Subsequently, a pattern of four strain gauges eventually forming abridge circuit is exposed and undergoes etching.

In this way, a load cell comprising four strain gauges 4 formed on thesurface of the electrically insulating film 2 of 2 μm overlaying thesurface of the strain inducing element is completed, each of said straingauges 4 being made of the copper-nickel alloy, as shown in (III) inFIG. 1.

In addition to the four strain gauges 4, as shown in FIG. 3, preciseresistors 5 for the zero-point adjustment if required are formed and,further, terminals 6 and patterned leads 7 are formed so that a bridgecircuit can be formed by connecting the strain gauges 4, the preciseresistors 5 for the zero-point adjustment and the terminals 6 throughthe patterned leads 7. In this way, the load cell is completed.

In the meantime, of the manufacturing steps referred to above, byvarying the composition ratio of the alloy used as material for thetarget during the sputtering process, that is, the weight ratio betweenthe copper and the nickel, a plurality of strain gauges having thecopper-nickel alloy films of different compositions were prepared and,then, the relationship between the copper-nickel composition and thetemperature dependent coefficient of resistance was measured while thestrain gauges were integrated together with the strain inducing element,the result of the measurement being shown in (I) in FIG. 4. In thisinstance, the coefficient of linear expansion of the strain inducingelement 1 made of aluminum alloy is 21.8×10⁻⁶ /deg.

In other words, the strain gauge made of the copper-nickel alloycontaining 55 wt % of copper and 45 wt % of nickel has exhibited atemperature dependent coefficient of resistance of about -70 PPM/degand, as the content of the nickel increases, the absolute value thereofdecreases. If the content of the nickel is further increases, thetemperature dependent coefficient of resistance turns to assume apositive sign.

Though even the strain gauge pattern formed in the above describedmanner exhibits the constant temperature dependent coefficient ofresistance, the resistance value itself varies with a passage of timeand, therefore, an aging is carried out for some hours at a temperatureof 150° to 200° C. under an oxygen-free atmosphere such as an atmospherecontaining argon or nitrogen. By so doing, the change in resistancevalue with time can be considerably minimized and, on the other hand,the temperature dependent coefficient of resistance relative of the samecomposition is shifted to positive side, as shown in (II) in FIG. 4, ascompared with that before the heat-treatment.

Where the heat-treatment is effected, based on the result shown in (II)in FIG. 4, it can be determined that the composition with which thetemperature dependent coefficient of resistance can be considered apractically zero value, that is, a value not greater than 6 PPM/deg, isthe composition within the range of 47.7 wt % of copper and 52.3 wt % ofnickel to 48.3 wt % of copper and 51.7 wt % of nickel. In other words,within the range of these weight ratios of the copper and the nickel,the coefficient of linear expansion of the strain inducing element andthe temperature dependent coefficient of resistance of the strain gaugepattern counterbalance with each other, resulting in that thetemperature dependent coefficient of resistance of the strain gaugepattern 4 which undergoes shrinkage and expansion together with thestrain inducing element is substantially zeroed.

In other words, when the strain gauges manufactured with the employmentof the copper-nickel alloy of a composition containing 48 wt % of copperand 52 wt % of nickel with which the temperature dependent coefficientof resistance can be considered substantially zero as discussed abovewas tested to determine the rate of change in resistance at atemperature range of 20° C. (reference temperature) ±30° C. (the ratiobetween the resistance value at the reference temperature and the amountof change), the rate of change in resistance in the order of 0 to 3×10⁻⁴has been shown as shown by (I) in FIG. 5. On the other hand, in the caseof the strain gauge (II) of the composition containing 44.2 wt % ofcopper and 55.8 wt % of nickel and the strain gauge (III) of acomposition containing 55.7 wt % of copper and 44.3 wt % of nickel, therespective rates of change in resistance have exceeded ±20×10⁻⁴.

From the foregoing, where the heat-treatment is to be employed, has beenjudged that the temperature dependent coefficient resistance of the loadcell as a whole can be minimized strain gauge is manufactured employingthe metal film formed by the use of the high frequency magnetronsputtering technique that the metal film can have a composition from thecomposition containing 47.7 wt % of copper an 52.3 wt % of nickel to thecomposition containing 48.3 wt % of and 51.7 wt % of nickel.

It is to be noted that, although in the illustrated embodiment a targetS which has been alloyed to have the predetermined composition ratio hasbeen employed, the composition ratio of the strain gauge can beadjusted, as shown in FIG. 6, even if the target S prepared (I) byplacing nickel chips 21 on one surface of a plate member 20 made of ametal which used as a base, for example, copper or a copper alloy, (II)placing a plate member 22 of copper and a plate member 23 of nickelseparately, or (III) by varying the surface area each of regions ofcopper 24 and nickel 25 forming a substrate.

Then, the load cell was prepared by forming a heat resistant film of,for example, polyimide on the strain inducing element made of stainlesssteel having a coefficient of linear expansion of 17.3×10⁻⁶ /deg,forming a resistance film on one surface of the insulating film whilethe composition ratio of copper and nickel is varied in a manner similarthat described hereinabove, forming a strain gauge pattern by etchingthe resistance film, and heat-treating it under the oxygen-freeatmosphere, and, thereafter, the composition ratio and the temperaturedependent coefficient of resistance of the resistance film was examined.As a result thereof, it has been found that, in the case of thecomposition ranging from the composition containing 47.2 wt % of copperand 52.8 wt % of nickel to the composition containing 47.8 wt % ofcopper and 52.2 wt % of nickel, the temperature dependent coefficient ofresistance exhibited by the load cell is substantially zero.

Also, although not included in the present invention, the load cell wasprepared by forming a heat resistant insulating film of, for example,polyimide on a glass substrate (corresponding to the strain inducingelement) having a coefficient of linear expansion of 4.6×10⁻⁶ /deg,forming a resistance film on one surface of the insulating film whilethe composition ratio of copper and nickel is varied in a manner similarto that described hereinabove, forming a strain gauge pattern by etchingthe resistance film, and heat-treating it under the oxygen-freeatmosphere, and, thereafter, the composition ratio and the temperaturedependent coefficient of resistance of the resistance film was examined.As a result thereof, it has been found that, in the case of thecomposition ranging from the composition containing 46.0 wt % of copperand 54.0 wt % of nickel to the composition containing 46.6 wt % ofcopper and 53.4 wt % of nickel, the temperature dependent coefficient ofresistance exhibited by the load cell is substantially zero.

In the meantime, as hereinbefore described, the coefficient of linearexpansion of the strain inducing element made of the aluminum alloy is21.8×10⁻⁶ /deg, the coefficient of linear expansion of the straininducing element made of the stainless steel is 17.3×10⁻⁶ /deg, and thecoefficient of linear expansion of the glass substrate is 4.6×10⁻⁶ /deg.In view of this, when the composition ratio of copper and nickel withwhich the strain gauge integrated with the strain inducing element canexhibit the smallest temperature dependent coefficient of resistance wasexperimentally determined in terms of the content (wt %) of nickel, thefollowing result was given. ##EQU2## If the nickel content is within therange determined by the above relationship equation to which ±0.3 wt %is added, that is, the above relationship equation (the value of whichis about 50) multiplied by (1±0.006), the load cell which one can regardthat, with respect to the temperature gradient produced in the weighingapparatus, the temperature dependent error is practically zero can berealized.

In order to ascertain the reliability of the above relationshipequation, in the case where the strain gauge is to be fabricated byforming a film of a copper-nickel alloy on a surface of a straininducing element made of brass having a coefficient of linear expansionof 20.8×10⁻⁶ /deg, the composition of the copper-nickel alloy whichwould result in a zero value of the temperature dependent coefficient ofresistance of the strain gauge when the latter is used in the load cellis examined and, as a result, data were obtained indicating that thecomposition is within the range of 47.6 wt % of copper and 52.4 wt % ofnickel to 48.2 wt % of copper and 51.8 wt % of nickel.

When the temperature dependent coefficient of resistance of the straingauge formed on a substrate of brass by the previously described methodon the basis of these data were measured and, as a result thereof, ameasurement of 5 PPM/deg was obtained.

In view of the foregoing, it has been ascertained that the foregoingrelationship equation has an extremely high reliability.

Furthermore, the concept of the present invention can be equallyapplicable to the load cell utilizing the strain inducting element madeof phosphoric bronze or any other metal such as a non-stainless lowalloyed steel, for example, nickel-chromium-molybdenum steel, or acarbon steel. Even in such case, the composition ratio of copper andnickel (weight ratio) with which the load cell can exhibit a zerotemperature dependent coefficient of resistance can be determined in amanner similar to that discussed hereinbefore. Although the content (wt%) of nickel determined in this way may differ from the value obtainedby the foregoing relationship equation, all that is to be done is thatthe weight ratio of copper and nickel suffices to be set to a value sodetermined in the previously described manner, that is, a value at whichthe temperature dependent coefficient of resistance of the strain gaugepattern which undergoes shrinkage and expansion together with the straininducing element can be substantially zeroed.

It is to be noted that, although in the foregoing embodiment the straingauge pattern has been described as formed by etching the resistancefilm formed of the copper-nickel alloy, similar effects can be obtainedeven if the strain gauge pattern is formed during the sputtering.

Also, although in the foregoing embodiment reference has been made tothe strain gauges formed on the strain inducing element of a blockconfiguration, similar effects can be obtained even if it is applied tothe load cell in which the strain gauges are formed on a strain inducingelement 1 of a plate-like configuration as shown in FIG. 7.

Again, although in the foregoing embodiment reference has been made tothe four strain gauges formed on the same strain inducing element,similar effects can be equally obtained even if the load cell isfabricated wherein only two strain gauges are formed on the straininducing element while the bridge circuit is formed with the use ofprecise fixed resistors on two sides.

Yet, although in the foregoing embodiment the resistance film for theformation of the strain gauges has been described as formed on a surfaceof the electrically insulating film formed on the surface of the straininducing element, it is obvious that the step of forming theelectrically insulating film can be dispensed with if the surface of thestrain inducing element is formed with an electrically insulating filmsuch as an oxide film.

As hereinbefore fully described, since in the practice of the presentinvention, the weight ratio of copper and nickel contained in the straingauge pattern comprising the film of the copper-nickel alloy formed bythe use of the sputtering technique is chosen to be of a value effectiveto permit the temperature dependent coefficient of resistance of thestrain gauge pattern to be substantially zero, that is, effective tocounterbalance the change in resistance resulting from the shrinkage andexpansion of each strain gauge due to the linear expansion of the straininducing element and the temperature dependent coefficient of thecopper-nickel alloy used to form the strain gauges with each other, thechange in resistance resulting from a change in temperature of any oneof the strain inducing element and the strain gauges can be minimizedand, therefore, even where applied to a subject of interest to bemeasured in which a temperature gradient occurs in the load cell, theload can be measured with extremely high precision.

INDUSTRIAL APPLICABILITY

The load cell to which the present invention concerns can be utilizednot only in a weighing apparatus such as, for example, an electronicscale, but also in an output torque measuring device and anaccelerometer used in connection with a power generating device such asan engine.

We claim:
 1. A load cell comprising a copper and nickel alloy strain gauge pattern formed on a metallic strain inducing element characterized in that the weight ratio or copper and nickel contained in the strain gauge pattern is chosen to be of a value effective to permit the temperature dependent coefficient of resistance of the strain gauge pattern which undergoes shrinkage and expansion together with the strain inducing element to be substantially zero, and is chosen such that the coefficient of linear expansion of the strain inducing element and said temperature dependent coefficient of resistance of the strain gauge pattern counterbalance each other.
 2. The load cell as defined in claim 1, wherein the strain inducing element is made of a metal selected from the group consisting of aluminum alloy, stainless steel, brass, phosphoric bronze, low alloyed steel and carbon steel. 